1. Field of the Invention
The present invention relates to a small-diameter, round fiber-reinforced plastic strand, a manufacturing method thereof, and a fiber-reinforced sheet in which such fiber-reinforced plastic strands are arranged in a sheet shape. Particularly, the fiber-reinforced sheet is applicable for bonding, for reinforcement, to, for example, a concrete structure or a steel structure which is a civil-engineering structure (in the present specification, such structures including a concrete structure and a steel structure are simply referred to as “structure”).
2. Description of the Related Art
For the purpose of reinforcing a structure, a bonding process of attaching or winding continuous fiber-reinforced sheets onto or around the surface of an existing or a new structure has recently been developed.
This bonding process is however only simple bonding. A limitation is imposed on reinforcing effect of the ultimate bearing force because of early destruction of structure caused by peeling of FRP (fiber-reinforced plastics) reinforcing members, and furthermore, the cracking inhibiting effect of, for example, a concrete structure has its limits. In addition, high performance of FRP reinforcing materials is not effectively utilized in many cases. Cracking damages in existing structures cannot be restored or structures cannot be reinforced against a dead load.
To solve these problems, a process known as the strained bonding process is now applied in practice, comprising the steps of achieving a tensioned state by inputting a load onto a sheet-shaped reinforcing member, and bonding the sheet-shaped reinforcing member in the tensioned state to the structure surface. The sheet-shaped reinforcing member used in this strained bonding process is a sheet in which fibers not impregnated with a resin are arranged in a direction, known as a reinforcing fiber sheet, or a fiber-reinforced plastic flat plate having a width lager than 50 mm.
However, in a fiber-reinforced sheet using fibers not impregnated with a resin, the reinforcing fibers are not always arranged uniformly in a direction because of the problems in manufacture or in handling. As a result, partial thread breakage occurs upon tensioning a fiber-reinforced sheet by imparting a load to introduce a tensioning force, and this may prevent from introducing a sufficient tensioning force. In other words, the fiber-reinforced sheet cannot sometimes display a force necessary for tensioning. The tensioning force is usually reduced by about 50 to 30% of the final breaking force.
When using a fiber-reinforced plastic flat plate, a large sheet width tends to cause a problem upon bonding in that it is difficult to obtain a sufficient adhesive strength due to mixture of voids onto the joint surface. In order to avoid occurrence of voids, a conceivable counter-measure is to provide holes in the fiber-reinforced plastic sheet. This is not, however, preferable since holes would cut reinforcing fibers of the fiber-reinforced plastic flat plate.
The present inventors have therefore proposed a fiber-reinforced sheet, as disclosed in Japanese patent application laid-open No. 2004-197325, in which a plurality of continuous fiber-reinforced plastic strands, impregnated with a matrix resin and cured, are arranged in the longitudinal direction in a blind shape, and these strands are then fixed by a fixing fiber material.
Such a fiber-reinforced sheet permits solving the problem of thread breakage upon tensioning, avoidance of occurrence of voids upon execution, thereby ensuring a sufficient adhesive strength for the surface to be reinforced, and particularly makes it possible to accomplish reinforcement of a concrete structure based on the strained bonding process at a very high workability.
It has been the usual and conventional practice to manufacture a round continuous fiber-reinforced plastic strand used in the aforementioned fiber-reinforced sheet by the application of a pultrusion forming process known as the pultrusion process.
In this process, however, a problem is encountered in that use of a heating die for forming a round strand causes generation of resistance during passage of strands through the die, thus preventing from raising the forming speed. Another problem is that the size limitation of die imposes a limit in the number of holes to be provided in the die, limiting the number of strands capable of being manufactured at a time to 20 to 30 strands.
The process suffers from still another problem in that a release agent is used for preventing sticking of the matrix resin and the die, and outflow of the release agent onto the strand surface prevents smooth bonding of the adhesive and the strand surface when bonding the strand to a structure, when using the strand as a sheet-shaped reinforcing member (fiber-reinforced sheet) for reinforcing the structure. This causes further another problem that, when using the fiber-reinforced sheet, surface roughing should be carried out with sand paper or the like after curing of the strand.
These problems have caused an increase in the manufacturing cost, and a decrease in quality due to flawing of strands upon surface roughing.